Here's the Thinkpad T20 service manual: support page, direct pdf link. The manual also covers the T21/22/23 models.
OP, grab a copy of this (although it sounds like you already have it).
You might get lucky and fix your issue by replacing the CMOS battery. If not, this service manual will help you do a partial/complete teardown of the system. You'll want to first try reseating the connectors for all the peripherals (RAM, network, drives); if that doesn't help, try to get to the CPU and LCD connectors.
From there, try disconnecting the non-critical components -- network and drives particularly. If you're still not getting anywhere, try swapping in parts if available, particularly RAM and CPU.
If you get to this point, you need to examine whether it's worth trying to fix. You might be better off finding another T20 (dead or alive) and try combining parts from both machines into one working system.